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Internet Engineering Task Force (IETF) L. Berger
Request for Comments: 5710 LabN
Category: Standards Track D. Papadimitriou
ISSN: 2070-1721 Alcatel Lucent
JP. Vasseur
Cisco
January 2010
PathErr Message Triggered MPLS and GMPLS LSP Reroutes
Abstract
This document describes how Resource ReserVation Protocol (RSVP)
PathErr messages may be used to trigger rerouting of Multi-Protocol
Label Switching (MPLS) and Generalized MPLS (GMPLS) point-to-point
Traffic Engineering (TE) Label Switched Paths (LSPs) without first
removing LSP state or resources. Such LSP rerouting may be desirable
in a number of cases, including, for example, soft-preemption and
graceful shutdown. This document describes the usage of existing
Standards Track mechanisms to support LSP rerouting. In this case,
it relies on mechanisms already defined as part of RSVP-TE and simply
describes a sequence of actions to be executed. While existing
protocol definitions can be used to support reroute applications,
this document also defines a new reroute-specific error code to allow
for the future definition of reroute-application-specific error
values.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5710.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................4
2. Reroute Requests ................................................4
2.1. Processing at Requesting Node ..............................4
2.1.1. Reroute Request Timeouts ............................5
2.2. Processing at Upstream Node ................................6
2.3. Processing at Ingress ......................................6
3. Example Reroute Requests ........................................7
3.1. Node Reroute Request .......................................7
3.2. Interface Reroute Request ..................................7
3.3. Component Reroute Request ..................................8
3.4. Label Reroute Request ......................................9
4. IANA Considerations .............................................9
5. Security Considerations ........................................10
6. References .....................................................10
6.1. Normative References ......................................10
6.2. Informative References ....................................11
7. Acknowledgments ................................................11
1. Introduction
The Resource ReserVation Protocol (RSVP), see [RFC2205], has been
extended to support the control of Traffic Engineering (TE) Label
Switched Paths (LSPs) for both Multi-Protocol Label Switching (MPLS)
and Generalized MPLS (GMPLS) in, respectively, [RFC3209] and
[RFC3473]. In all cases, a PathErr message is used to report errors
to nodes upstream of the error-detecting node. As defined in
[RFC2205] and left unmodified by [RFC3209], PathErr messages "do not
change path state in the nodes through which they pass".
Notwithstanding this definition, PathErr messages are most commonly
used to report errors during LSP establishment, i.e., the RSVP-TE
processing that occurs prior to the ingress receiving a Resv message.
(See [RFC5711] for a broader discussion on PathErr message handling.)
Support for such usage was enhanced via the introduction of the
Path_State_Removed flag in [RFC3473], which enables a processing node
to free related LSP state and resources. The usage of PathErr
messages during LSP establishment was further covered in [RFC4920],
which describes in detail how a node may indicate that it or one of
its associated resources should be avoided, i.e., routed around,
during LSP establishment.
PathErr messages can also be used to support a number of other cases
that can occur after an LSP is established. This document focuses on
the cases where PathErr messages can be used for a node to indicate
that it desires an upstream node to reroute an LSP around the
indicating node or resources associated with the indicating node.
Some examples of such cases are soft-preemption and graceful shutdown
(see [RFC5712] and [GRACEFUL]).
This document uses the terminology "reroute request" to refer to the
indication by a node that an upstream reroute should take place.
This document describes how a node can initiate a reroute request
without disrupting LSP data traffic or, when so desired, with the
disruption of data traffic and removal of LSP-associated state and
resources. The applicability of this document is limited to point-
to-point LSPs. Support for point-to-multipoint LSPs are for further
study.
The mechanisms used to indicate reroute requests are derived from the
mechanisms described in [RFC4920] and the error codes defined in
[RFC4736]. This document describes (1) how a non-disruptive reroute
request may be issued and, (2) based on an optional "timeout" period,
how rerouting may be forced by removing LSP state and associated
resources and signaling such removal. While this document describes
how existing protocol definitions can be used to support rerouting,
it also defines a new reroute-specific error code to allow for the
future definition of reroute-application-specific error values.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Reroute Requests
This section describes how a downstream node can indicate that it
desires a node upstream (along the LSP path) to initiate the
rerouting of an LSP, and how the upstream nodes can respond to such a
request. Initiating nodes, transit nodes, and ingress nodes are
described separately.
2.1. Processing at Requesting Node
When a transit or egress node desires to request the rerouting of an
established LSP, it first determines if it can act on the reroute
request locally. Such a check MUST be performed on the condition
that the Explicit Route Object (ERO), see [RFC3209], received in the
LSP's incoming Path message does not preclude LSP rerouting.
Examples of requests that may trigger reroutes are avoiding an
outgoing interface, a component, label resource, or a next hop not
explicitly listed in the ERO. In all cases, the actual repair action
SHOULD be performed after verification that the local policy allows
local repair for that LSP/state. That is, any traffic-rerouting
action (associated to this state) must be initiated and completed
only as allowed by local node policy.
When the node cannot act locally, it MUST issue a PathErr message
indicating its inability to perform local rerouting. The PathErr
message MUST contain an ERROR_SPEC object of the format defined in
[RFC2205] or [RFC3473]. Such a message MUST include one of the
following combinations of error codes and error values:
1. "Notify/Local node maintenance required" to support backwards
compatibility and to reroute around the local node.
2. "Notify/Local link maintenance required" to support backwards
compatibility and to reroute around a local interface.
3. "Reroute/<any Reroute error value>" for future compatibility
and when backwards compatibility is not a concern.
The rest of the ERROR_SPEC object is constructed based on the local
rerouting decision and the resource that is to be avoided by an
upstream node. It is important to note that the address and TLVs
carried by the ERROR_SPEC object identify the resource to be avoided
and not the error code and value.
When the reroute decision redirects traffic around the local node,
the local node MUST be indicated in the ERROR_SPEC object.
Otherwise, i.e., when the reroute decision does not redirect traffic
around the local node, the impacted interface MUST be indicated in
the ERROR_SPEC object and the IF_ID [RFC3473] ERROR_SPEC object
formats SHOULD be used to indicate the impacted interface.
The IF_ID [RFC3473] ERROR_SPEC object format MUST be used to indicate
a reroute request that is more specific than an interface. The TLVs
defined in [RFC3471], as updated by [RFC3477], [RFC4201], and
[RFC4920] MAY be used to provide specific, additional reroute request
information, e.g., reroute around a specific label. The principles
related to ERROR_SPEC object construction, defined in Section 6.3.1
of [RFC4920], SHOULD be followed.
2.1.1. Reroute Request Timeouts
Reroute request timeouts are used to remove an LSP when there is no
response to a reroute request. A reroute request timeout is used
when an LSP is to be removed at the expiration of the reroute request
timeout period. When such LSP removal is desired, and after
initiating a reroute request, the initiating node MUST initiate a
timeout during which it expects to receive a response to the reroute
request. Valid responses are a PathTear message or a trigger Path
message with an ERO, avoiding the resource that was indicated in the
reroute request. If either type of message is received, the timeout
period MUST be canceled and no further action is needed. Note,
normal refresh processing is not modified by the introduction of
reroute request timeouts. Such processing may result in Path state
being removed during the timeout period, in which case the timeout
period MUST also be canceled.
If the reroute request timeout is reached, the initiating node MUST
remove the LSP and its associated state and resources. Removal of
LSP state is indicated downstream via a corresponding PathTear
message. Removal is indicated upstream via a PathErr message with
the error code of "Service preempted". The Path_State_Removed flag
MUST be set if supported. When the Path_State_Removed flag is not
supported, a corresponding ResvTear MUST also be sent.
2.2. Processing at Upstream Node
When a transit node's policy permits it to support reroute request
processing and local repair, the node MUST examine incoming PathErr
messages to see it the node can perform a requested reroute. A
reroute request is indicated in a received PathErr message, which
carries one of the error code and value combinations listed above in
Section 2.1. Note that a conformant implementation MUST check for
any of the three combinations listed in Section 2.1.
A transit node MAY act on a reroute request locally when the ERO
received in the LSP's incoming Path message does not preclude the
reroute. As before, examples include loosely routed LSP next hops.
When the reroute request can be processed locally, standard, local
repair processing MUST be followed. The node SHOULD limit the number
of local repair attempts. Again, the expected norm is for local
repair, and thereby this case, to be precluded due to policy.
When the transit node supports [RFC4920] and is a boundary node, and
Boundary rerouting is allowed, it SHOULD use a route request as a
trigger to reroute the LSP. (Per [RFC4920], the Flags field of the
LSP_ATTRIBUTES object of the initial Path message indicates "Boundary
rerouting".) In the case the node triggers rerouting, it first MUST
identify an alternate path within the domain. When such a path is
available, the node MUST terminate the PathErr message and issue a
Path message reflecting the identified alternate path. Processing
then continues per [RFC4920]. When an alternate path is not
available, the node cannot act on the reroute request.
When a transit node cannot act on a reroute request locally, per
standard processing, it MUST propagate the received PathErr message
to the previous hop.
2.3. Processing at Ingress
When reroute processing is supported, an ingress node MUST check
received PathErr messages to identify them as indicating reroute
requests. A reroute request is indicated in a received PathErr
message, which carries one of the error code and value combinations
listed above in Section 2.1. Note that a conformant implementation
MUST check for any of the three combinations listed in Section 2.1.
Upon receiving a reroute request, the ingress MUST attempt to
identify an alternate path, avoiding the node, interface, resource,
etc. identified within the ERROR_SPEC object. When an alternate path
cannot be identified, the reroute request MUST be discarded. When an
alternate path is identified, a corresponding make-before-break LSP
SHOULD be initiated and standard make-before-break procedures MUST be
followed.
3. Example Reroute Requests
This section provides example reroute requests. This section is
informative rather than prescriptive. Reroute requests are always
sent via PathErr messages. As described above, a PathErr message may
contain either an [RFC2205] format ERROR_SPEC object, or an IF_ID
[RFC3473] format ERROR_SPEC object; it is the address and TLVs
carried by the ERROR_SPEC object, and not the error value, that
indicates the resource that is to be avoided by the reroute.
3.1. Node Reroute Request
To indicate that the node should be avoided by an upstream node, the
node originating the reroute may format the ERROR_SPEC per [RFC2205],
for example:
o IPv4 ERROR_SPEC object: Class = 6, C-Type = 1
+-------------+-------------+-------------+-------------+
| IPv4 Error Node Address (4 bytes) |
+-------------+-------------+-------------+-------------+
| Flags | Error Code | Error Value |
+-------------+-------------+-------------+-------------+
The node address is set to the local node's TE router address. Error
code is set to either "Notify/Local node maintenance required" or
"Reroute/<any Reroute error value>".
3.2. Interface Reroute Request
To indicate that a numbered interface should be avoided by an
upstream node, the node originating the reroute may format the
ERROR_SPEC per [RFC3473], for example:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Class-Num (6) | C-Type (3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Error Node Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Error Code | Error Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1) | Length (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The node address is set to the local node's TE router address. Error
code is set to either "Notify/Local link maintenance required" or
"Reroute/<any Reroute error value>". IP address is set to the TE
address of the interface to be avoided.
3.3. Component Reroute Request
To indicate that an unnumbered component should be avoided by an
upstream node, the node originating the reroute formats the
ERROR_SPEC per [RFC4201], for example:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Class-Num (6) | C-Type (3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Error Node Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Error Code | Error Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (3) | Length (12) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface ID (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The node address is set to the local TE address used in the
advertisement of the bundle associated with the component. Error
code is set to either "Notify/Local link maintenance required" or
"Reroute/<any Reroute error value>". Router ID is set to the local
router ID, and Interface ID is the identifier assigned to the
component link by the local node.
3.4. Label Reroute Request
To indicate that a label should be avoided by an upstream node, the
node originating the reroute may format the ERROR_SPEC per [RFC4920],
for example:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Class-Num (6) | C-Type (3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Error Node Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Error Code | Error Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1) | Length (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (6) | Length (8) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DOWNSTREAM_LABEL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The node address is set to the local node's TE router address. Error
code is set to either "Notify/Local link maintenance required" or
"Reroute/<any Reroute error value>". IP address is set to the TE
address of the interface that supports the label to be avoided.
DOWNSTREAM_LABEL indicates the label to be avoided.
4. IANA Considerations
IANA assigned values for namespaces defined in this document and
reviewed in this section.
IANA made the assignment in the "Error Codes and Globally-Defined
Error Value Sub-Codes" section of the "RSVP Parameters" registry:
34 Reroute [RFC5710]
This error code has the following defined Error Value sub-code:
0 = Generic LSP reroute request
Reroute error values should be allocated based on the following
allocation policy as defined in [RFC5226].
Range Registration Procedures
-------- ------------------------
0-32767 IETF Consensus
32768-65535 Private Use
5. Security Considerations
Sections 9 of [RFC4920] and [RFC4736] should be used as the starting
point for reviewing the security considerations related to the
formats and mechanisms discussed in this document. This document
introduces a new error code, but this code is functionally equivalent
to existing semantics, in particular, the error code/error value
combinations of "Notify/Local node maintenance required" and
"Notify/Local link maintenance required". As such, this document
introduces no new security considerations beyond what already applies
to these existing formats and mechanisms. Future documents may
define new error values; any considerations specific to those values
should be discussed in the document defining them.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2205] Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and
S. Jamin, "Resource ReSerVation Protocol (RSVP) --
Version 1 Functional Specification", RFC 2205, September
1997.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan,
V., and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC3471] Berger, L., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Functional Description", RFC
3471, January 2003.
[RFC3473] Berger, L., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation
Protocol-Traffic Engineering (RSVP-TE) Extensions", RFC
3473, January 2003.
[RFC3477] Kompella, K. and Y. Rekhter, "Signalling Unnumbered
Links in Resource ReSerVation Protocol - Traffic
Engineering (RSVP-TE)", RFC 3477, January 2003.
[RFC4201] Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling
in MPLS Traffic Engineering (TE)", RFC 4201, October
2005.
[RFC4920] Farrel, A., Ed., Satyanarayana, A., Iwata, A., Fujita,
N., and G. Ash, "Crankback Signaling Extensions for MPLS
and GMPLS RSVP-TE", RFC 4920, July 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
6.2. Informative References
[RFC4736] Vasseur, JP., Ed., Ikejiri, Y., and R. Zhang,
"Reoptimization of Multiprotocol Label Switching (MPLS)
Traffic Engineering (TE) Loosely Routed Label Switched
Path (LSP)", RFC 4736, November 2006.
[GRACEFUL] Ali, Z., Vasseur, JP., Zamfir, A., and J. Newton,
"Graceful Shutdown in MPLS and Generalized MPLS Traffic
Engineering Networks", Work in Progress, September 2009.
[RFC5711] Vasseur, JP., Ed., Swallow, G., and I. Minei, "Node
Behavior upon Originating and Receiving Resource
Reservation Protocol (RSVP) Path Error Messages", RFC
5711, January 2010.
[RFC5712] Meyer, M., Ed. and JP. Vasseur, Ed., "MPLS Traffic
Engineering Soft Preemption", RFC 5712, January 2010.
7. Acknowledgments
This document was conceived along with Matthew Meyer. George Swallow
provided valuable feedback. The General Area Review Team (Gen-ART)
review was performed by Francis Dupont.
Authors' Addresses
Lou Berger
LabN Consulting, L.L.C.
Phone: +1-301-468-9228
EMail: lberger@labn.net
Dimitri Papadimitriou
Alcatel Lucent
Francis Wellesplein 1,
B-2018 Antwerpen, Belgium
Phone: +32 3 240-8491
EMail: Dimitri.Papadimitriou@alcatel-lucent.be
JP Vasseur
Cisco Systems, Inc
11, Rue Camille Desmoulins
L'Atlantis
92782 Issy Les Moulineaux
France
EMail: jpv@cisco.com

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